This invention relates to devices for controlling or limiting differences in angular velocity or angular acceleration between relatively rotatable members. Hereinafter such a device is referred to as "a control coupling". Control couplings are particularly useful in controlling the relative rotation between parts of a differential gear and particularly in controlling such a differential gear when used as an interaxle differential in a four-wheel drive vehicle.
More particularly, the invention is concerned with a control coupling, hereinafter referred to as being of the kind specified, comprising mutually interleaved elements arranged for rotation about a common axis within an enclosure containing a viscous liquid in contact with adjacent working surfaces of said elements, alternate ones of said elements constituting a first set of elements for driving connection with a first rotatable member and remaining ones of said elements constituting a second set of elements for driving connection with a second rotatable member. In such a coupling, the liquid within the enclosure and the construction thereof will be such that, at a predetermined working temperature of at least 100.degree. C., the pressure of the liquid in the enclosure rises substantially above ambient for the reasons explained below.
When such a coupling is in use, energy will be dissipated in the coupling which will cause a rise in temperature thereof. The rise in temperature causes expansion of the liquid and the co-efficient of thermal expansion of the liquid is so much greater than that of the material of the coupling that, at a predetermined temperature, the liquid substantially fills the space available for fluid within the coupling enclosure. When the liquid fills the coupling we have found that the pressure of liquid within the coupling also rises.